with as open shutter of 303 seconds, I'm guessing you just set it on "bulb", pressed the shutter, walked away, and came back after you remembered it was going... ...at least I would have forgotten about it for a while.

pretty much like that yeah, the longest shutter speed on my camera is 30 seconds, so I used the little remote thing and set it (its just like bulb, but it doesnt say bulb), made some cocoa actually, then stopped it when I felt enough time had elapsed.